1. Field of the Invention
The present invention relates to a method for developing an electrostatic latent image formed on the surface of a photosensitive body (for example, the photosensitive drum of the general type used in photocopy machines, laser printers, etc.) with one-component developers.
2. Description of the Prior Art
It is known to use one-component developers and two-component developers for developing latent electrostatic images.
A two-component developer includes toner particles which actually form a visible image from the latent image and carrier particles for properly charging the toner. In such two-component developers, a mixing ratio of toner to carrier, i.e., a toner concentration, must be kept constant. However, it is difficult to maintain toner concentration constant. One-component developer has an advantage in that concentration control is not necessary since toner is the only component of the developer.
One-component developers are generally classified into two distinct types, namely magnetic and non-magnetic developers. Magnetic developers use magnetic materials together with non-magnetic materials as the developer particles. When such a magnetic developer is used in a conventional apparatus, a magnet is positioned inside a developer carrier for carrying developer to the developing position and generating a magnetic field for supporting and carrying the developer. However, the following problems exist in using magnetic developer.
First, the developer carrier must be a fairly complicated structure that is large in size. It is expensive to produce because the magnet must be supported by the developer carrier. Second a magnetic developer containing magnetic particles is more expensive than a non-magnetic developer. Finally, since the magnetic particles do not contribute to development, color reproducibility is not satisfactory. As a result, it is difficult to perform color development using a magnetic developer.
In an effort to overcome these problems, a developing apparatus using a non-magnetic one-component developer was developed, as disclosed in U.S. Pat. No. 4,521,098 to Hosoya et al. In Hosoya et al, a thin film layer of a non-magnetic toner is formed on a developing roller. It is pressed by only one blade, and the toner is triboelectrically charged by the blade. Thereafter, the toner is supplied to a photosensitive body on which a latent image is formed.
The concept of using a one-component developer having toner particles of an average particle diameter of 5-30 .mu.m is disclosed by U.S. Pat. No. 4,342,822. However, the developing method disclosed in that patent does not specify an appropriate average particle diameter. Average particle diameter is significant with respect to the quality of development. When the toner particles have an average particle diameter that is too large, images are not developed with a high resolution. Further, when the toner particles have an average particle diameter that is too large, it is not sufficiently charged. When the toner particles have an average particle diameter that is too small, the toner does not flow well through the system in which it is used.
With the conventional developing apparatus, because toner is triboelectrically charged on only one blade, all the toner particles cannot be sufficiently and reliably charged. When the toner is insufficiently charged, fogging or scattering of toner particles occurs, and a clear image cannot be obtained. When a toner image is successively transferred onto sheets, transferred images become blurred due to insufficient charging of toner.
Other background information is disclosed in the following documents: "XEROGRAPHIC DEVELOPMENT USING SINGLE-COMPONENT NON-MAGNETIC TONER" by Masahiro Hosoya, Shinya Tomura, and Tsutomu Uehara of the Toshiba R & D Center; published by the IEEE in 1985; U.S. Pat. No. 3,731,146 -- Bettiga et al (May 1, 1973), U.S. Pat. No. 4,498,756 -- Hosoya et al (Feb. 12, 1985), U.S. Pat. No. 4,656,965 -- Hosoya et al (Apr. 14, 1987), and U.S. Pat. No. 4,628,860 -- Hosoya et al (Dec. 16, 1986). The disclosures of the U.S. patent references are hereby incorporated by reference as if their disclosures were fully set forth herein.